System and Method for Loop Suppression in Transit Networks

ABSTRACT

An embodiment method of loop suppression in a layer-two transit network with multiprotocol label switching (MPLS) encapsulation includes receiving a packet at a provider edge (PE) router for the layer-two transit network. The packet is stored in a non-transitory memory on the PE router. The packet is stored according to a packet data structure having an MPLS label field and a layer-two header. A time-to-live (TTL) attribute is then determined for the packet. The TTL attribute is written to the non-transitory memory in the MPLS label field. The packet is then routed according to information in the layer-two header.

TECHNICAL FIELD

The present invention relates generally to loop suppression in transitnetworks and, in particular embodiments, to a system and method for loopsuppression in a layer-two transit network with multiprotocol labelswitching (MPLS) encapsulation.

BACKGROUND

The Open Systems Interconnection (OSI) model partitions communicationsystems into abstraction layers. A given layer serves layers above, andis served by layers below. For example, a first layer serves a secondlayer, and the second layer serves a third. The first layer is aphysical layer that defines physical specifications for a dataconnection. Physical specifications include connector layouts, cablespecifications, and hardware specifications, among others. The secondlayer is a data link layer that provides a reliable link between twodirectly connected nodes. For example, Ethernet is a layer-two protocolthat utilizes the physical layer to provide an Ethernet link between twonodes. The third layer is a network layer that provides procedures andfunctionality to define a network over which data sequences, i.e.,datagrams, are transmitted among various nodes in the network. Forexample, internet protocol (IP) is a layer-three protocol that providesmany capabilities, including routing functionality and IP addresses,among others.

One capability introduced in layer-three networks is loop suppression.IP introduces a time-to-live (TTL) attribute in an IP header thatencapsulates a given packet. The TTL attribute can be used as anindicator of a loop's existence in a network. The general idea is that apacket should be discarded, or dropped, by the network after a certainnumber of hops to prevent infinite unicast or multicast loops.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a system and method forloop suppression in transit networks.

An embodiment method of loop suppression in a layer-two transit networkwith multiprotocol label switching (MPLS) encapsulation includesreceiving a packet at a provider edge (PE) router for the layer-twotransit network. The packet is stored in a non-transitory memory on thePE router. The packet is stored according to a packet data structurehaving an MPLS label field and a layer-two header. A time-to-live (TTL)attribute is then determined for the packet. The TTL attribute iswritten to the non-transitory memory in the MPLS label field. The packetis then routed according to information in the layer-two header.

An embodiment method of Ethernet packet routing in a layer-two transitnetwork with MPLS encapsulation includes receiving a packet at aprovider router for the layer-two transit network. The packet has anMPLS label field and a layer-two header. The packet is stored in anon-transitory memory on the provider router. The provider routerevaluates a TTL attribute stored in the MPLS label field for loopdetection. When the TTL attribute indicates a loop, the packet isdropped. When the TTL attribute does not indicate a loop, the TTLattribute is recalculated and written to the non-transitory memory. Thepacket is then routed according to information in the layer-two header.

An embodiment PE router includes a network interface controller (NIC), anon-transitory memory, and a processor. The NIC is couplable to atransit network. The non-transitory memory is configured to store anMPLS encapsulated packet having an MPLS label field and a layer-twoheader. The processor is coupled to the non-transitory memory and theNIC. The processor is configured to compute a starting TTL value for theMPLS encapsulated packet. The processor is also configured to cause thestarting TTL value to be written to the MPLS label field in thenon-transitory memory. The processor is further configured to instructthe NIC to transmit the MPLS encapsulated packet according toinformation in the layer-two header.

An embodiment provider router includes a NIC, a non-transitory memory,and a processor. The NIC has a first port and a second port, bothcouplable to a transit network. The non-transitory memory is configuredto store an MPLS encapsulated packet. The MPLS encapsulated packet isreceivable through the first port and includes an MPLS label field alayer-two header. The processor is coupled to the NIC and thenon-transitory memory. The processor is configured to evaluate a TTLattribute stored in a memory block of the non-transitory memorycorresponding to the MPLS label field for loop detection. When the TTLattribute in the non-transitory memory indicates a loop, the MPLSencapsulated packet is dropped. When the TTL attribute does not indicatea loop, a new TTL value is computed and written to the memory blockcorresponding to the MPLS label field. The processor then causes the NICto transmit the MPLS encapsulated packet through the second portaccording to information in the layer-two header.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a transit network;

FIG. 2 is a block diagram of one embodiment of a provider router;

FIG. 3 is a block diagram of one embodiment of an MPLS label datastructure;

FIG. 4 is a flow diagram of one embodiment of a method of loopsuppression in a layer-two transit network with MPLS encapsulation;

FIG. 5 is a flow diagram of one embodiment of a method of Ethernetpacket routing in a layer-two transit network with MPLS encapsulation;and

FIG. 6 is a block diagram of a computing system.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of embodiments are discussed in detail below. Itshould be appreciated, however, that the present invention provides manyapplicable inventive concepts that may be embodied in a wide variety ofspecific contexts. The specific embodiments discussed are merelyillustrative of specific ways to make and use the invention, and do notlimit the scope of the invention.

Multiprotocol label switching (MPLS) is an encapsulation technique thatprovides additional Ethernet packet routing capabilities beyond Ethernetrouting. Ethernet routing utilizes data in a layer-two header, i.e., anEthernet header, to route a packet from node to node. MPLS applies alabel to the packet, e.g., a transit label or a service label,effectively encapsulating the packet. In certain embodiments, MPLSencapsulation uses a service label only, while, in other embodiments.MPLS encapsulation uses both a transit label and a service label.Hardware in a layer-three network typically includes an MPLS controlplane to facilitate packet forwarding according to the MPLS labels anddata in the layer-three header, i.e., the IP header. The transit labelidentifies a particular transit network as a destination for the packet.The transit network bridges two other networks, which can include one ormore service networks. The service label identifies a service network asa destination for the packet. A service network is a network of linkeddevices, such as a virtual private network (VPN) or a virtual privatelocal area network (VLAN), among others. Hardware in a layer-two networkgenerally lacks the MPLS control plane and therefore relies on Ethernetrouting for packet forwarding. Some layer-two networks incorporate MPLSencapsulation, but without the MPLS packet forwarding, and route withouta layer-three header. These are sometimes referred to aslayer-two-and-a-half networks.

It is realized herein that loop suppression can be achieved in alayer-two network without a need for additional hardware or packetheaders. In a layer-two transit network using MPLS encapsulation, packetforwarding is carried out according to data in the layer-two header,which can include a backbone media access control address (BMAC), adestination address (DA) and a virtual local area network identifier(VID). It is realized herein that a TTL attribute can be included in anMPLS label field of a MPLS encapsulated packet. The TTL attribute can beincluded in either a transit label field or a service label field, whichdepends at least partially on whether a given embodiment uses an MPLStransit label, an MPLS service label, or both. The MPLS label field isgenerally a 32-bit field having a 20-bit MPLS label portion, a 3-bitquality of service (QoS) portion, a 1-bit stack indicator portion, and aremaining 8 bits that can be used for the TTL attribute. In alternativeembodiments, the size of the MPLS label field and the allocation of bitsto the various portions can vary per implementation. For example, theTTL attribute can be allocated 7 bits, 6 bits, 5 bits, etc. In otherembodiments, the TTL attribute can be allocated more bits. For example,certain embodiments of the MPLS label can include 9 bits, 10 bits, 11bits, 12 bits, etc. for the TTL attribute. Additionally, alternativeembodiments can include fewer or additional portions.

By utilizing MPLS encapsulation and packing the TTL attribute in theMPLS label field, embodiment transit networks can achieve loopsuppression without additional hardware to handle additional labels andwithout the MPLS control plane for routing packets. Provider routers cancarry out packet forwarding via software configured to forward packetsbased on the layer-two header. It is further realized herein thatpacking the TTL attribute in the MPLS label field avoids utilizing otherfields in the packet, which generally reduces capability elsewhere inlayer-two protocols.

It is also realized herein that provider routers within the transitnetwork can extract the TTL attribute from the MPLS label field anddetermine whether a given packet should be forwarded or dropped. When apacket is not dropped, the provider routers can recompute the TTLattribute according to its value at arrival. The precise method ofcomputing a new value for the TTL attribute can vary amongimplementations. In certain embodiments, each recalculation can includea linear function of the value at arrival. For example, a simpleapproach is to use a starting TTL value equal to the maximum number ofhops allowed through a transit network. The starting value for the TTLattribute is computed by an ingress provider router, also referred to asa provider edge (PE) router or ingress PE router. As a packet moves fromone hop to the next, the provider router at that hop decrements the TTLvalue by one and forwards the packet. Provider routers generally includehardware and software configured to count hops and to carry out packetrouting. When the packet arrives at a provider router with a TTLattribute value of zero, the packet is dropped. Alternatively, when thepacket reaches an egress provider router, also referred to as a PErouter or an egress PE router, the TTL attribute is removed along withthe MPLS label.

In alternative embodiments, it is realized herein, the function by whicha new TTL attribute value is computed can be any linear function.Additionally, the starting and end values for the TTL attribute can bevaried to suit a given linear function and transit network.

FIG. 1 is a block diagram of a transit network 100. Transit network 100includes an ingress PE router 110, a core of provider routers 120-1through 120-8, and an egress PE router 130. Ingress PE router 110 andegress PE router 130 are also provider routers, although they arespecialized to carry out necessary functions at the edges of transitnetwork 100. A packet, transmitted from a source 140, enters transitnetwork 100 at ingress PE router 110. Ingress PE router 110 isconfigured to apply MPLS encapsulation to the packet. The MPLS labelfield for the packet is populated with a TTL attribute that is computedby ingress PE router 110. The starting value for the TTL attribute isrelated to the maximum number of hops necessary to traverse transitnetwork 100, which is sometimes referred to as the diameter of thenetwork. The packet is then forwarded to one of provider routers 120-1through 120-8. Transit network 100 has a diameter of five, because apacket can be routed from source 140 to a destination 150 in five hops.The maximum number of hops allowed before dropping a packet can beadjusted for a given transit network. For example, in alternativeembodiments, the maximum number of hops can be specified as the networkdiameter plus one. The additional margin can increase the robustness ofthe transit network without too much impact on network resources.

Provider routers 120-1 through 120-8 are configured to evaluate the TTLattribute in the MPLS label field of the packet. Provider routers 120-1through 120-8 check whether the TTL attribute value has reached athreshold that indicates a loop exists. When the TTL attribute indicatesa loop exists, the packet is dropped. Otherwise, the packet is forwardedaccording to information in the layer-two header. The TTL attributevalue is recomputed before the packet is forwarded, and reevaluated atthe next hop.

When the packet reaches egress PE router 130, the MPLS encapsulation isremoved, by removing the MPLS label from the packet. The packet is thenrouted to destination 150.

FIG. 2 is a block diagram of one embodiment of a provider router 200. Incertain embodiments, provider router is part of a core of providerrouters in a transit network. In alternative embodiments, providerrouter is a PE router. Provider router 200 includes a memory 210, anetwork interface controller (NIC) 220, and a processor 230. Memory 210,NIC 220, and processor 230 are coupled to a bus 240. Bus 240 facilitatesthe transfer of data and instructions among memory 210, NIC 220, andprocessor 230.

NIC 220 is a physical network interface that couples provider router 200to a transit network. NIC 220 is configured to receive packets. Memory210 is a non-transitory memory and is configured to store a packet datastructure 250. Packet data structure 250 includes a payload portion 252,a layer-two header 254, and an MPLS label 256.

In embodiments where provider router 200 is a PE router, processor 230is configured to apply MPLS encapsulation to a received packet stored inmemory 210. The MPLS encapsulation adds an MPLS label to the packet,which is stored in MPLS label 256 in packet data structure 250.Processor 230 is further configured to determine a starting value for aTTL attribute. The starting value is written to MPLS label 256 in packetdata structure 250. Processor 230 then instructs NIC 220 to route thepacket according to information in layer-two header 254.

In embodiments where provider router 200 is part of the core of providerrouters in the transit network, the packet received by NIC 220 is anMPLS encapsulated packet. Processor 230 is configured to extract the TTLattribute from MPLS label 256 in packet data structure 250. Processor230 is further configured to evaluate the TTL attribute and determinewhether the packet should be dropped. When the TTL attribute indicates aloop exists in the transit network, processor 230 causes the packet tobe dropped. When the TTL attribute does not indicate a loop exists,processor 230 determines a new value for the TTL attribute. The newvalue can be computed according to a linear function of the arrivalvalue of the TTL attribute. Processor 230 then instructs NIC 220 toforward the packet according to information in layer-two header 254.

Processor 230 can be implemented in one or more processors, one or moreapplication specific integrated circuits (ASICs), one or morefield-programmable gate arrays (FPGAs), dedicated logic circuitry, orany combination thereof, all collectively referred to as a processor.The functions for processor 230 can be stored as instructions innon-transitory memory for execution by processor 230.

FIG. 3 is a block diagram of one embodiment of an MPLS label datastructure 300. MPLS label data structure 300 includes an MPLS labelportion 310, a QoS portion 320, a stack indicator portion 330, and a TTLportion 340. In certain embodiments, the allocation of bits among theportions varies per implementation. For example, in an embodiment whereMPLS label data structure 300 includes 32 bits, MPLS label portion 310can be allocated 20 bits, QoS portion 320 can be allocated 3 bits, stackindicator 330 can be allocated 1 bit, and the remaining 8 bits can beallocated to TTL portion 340. In alternative embodiments, MPLS labeldata structure 300 can be allocated greater or fewer than 32 bits. Insome embodiments, MPLS label data structure 300 can include additionalportions among which the bits of MPLS label data structure 300 areallocated.

FIG. 4 is a flow diagram of one embodiment of a method of loopsuppression in a layer-two transit network with MPLS encapsulation. Themethod begins at a start step 410. At a receive step 420, a packet isreceived at a PE router. The received packet is stored at a storing step430. The packet is stored in a non-transitory memory according to apacket data structure. The packet data structure includes an MPLS labelfield and a layer-two header. In certain embodiments, the MPLS labelfield in the packet data structure is a service label field. In otherembodiments, the MPLS label field is a transit label field. In someembodiments, the packet data structure includes both the service labelfield and the transit label field.

At a computation step 440, a TTL attribute is determined for the packet.The starting value for the TTL attribute is determined according to themaximum number of hops needed to traverse the layer-two transit network.The TTL attribute is written to the non-transitory memory at a storingstep 450. The TTL attribute is stored in the MPLS label field of thepacket data structure. The packet is then routed at a routing step 460.Packet routing is carried out according to information in the layer-twoheader. The method then ends at an end step 470.

FIG. 5 is a flow diagram of one embodiment of a method of Ethernetpacket routing in a layer-two transit network with MPLS encapsulation.The method begins at a start step 510. At a receive step 520, a packetis received at a provider router. The packet is MPLS encapsulated andincludes an MPLS label field and a layer-two header. At a storing step530, the packet is stored in a non-transitory memory. The packet can bestored according to a packet data structure.

At an evaluation step 540, a TTL attribute is extracted from thenon-transitory memory and evaluated for loop detection. The TTLattribute is extracted from the MPLS label field of the packet. Adetermination is made at a check step 550 as to whether the TTLattribute indicates a loop exists in the layer-two transit network. Ifthe TTL attribute indicates a loop, then the packet is dropped at adropping step 560. Otherwise the method continues to a TTL recalculationstep 570. At TTL recalculation step 570, a new value for the TTLattribute is computed and written to the non-transitory memory in theMPLS label field. The packet is then routed at a routing step 580.Packet routing is carried out according to information in t thelayer-two header in the non-transitory memory. The method then ends atan end step 590.

FIG. 6 is a block diagram of a computing system 600 that may be used forimplementing the devices and methods disclosed herein. Specific devicesmay utilize all of the components shown or only a subset of thecomponents, and levels of integration may vary from device to device.Furthermore, a device may contain multiple instances of a component,such as multiple processing units, processors, memories, transmitters,receivers, etc. The computing system 600 may comprise a processing unit602 equipped with one or more input/output devices, such as a speaker,microphone, mouse, touchscreen, keypad, keyboard, printer, display, andthe like. The processing unit may include a central processing unit(CPU) 614, memory 608, a mass storage device 604, a video adapter 610,and an I/O interface 612 connected to a bus 620.

The bus 620 may be one or more of any type of several bus architecturesincluding a memory bus or memory controller, a peripheral bus, videobus, or the like. The CPU 614 may comprise any type of electronic dataprocessor. The memory 608 may comprise any type of system memory such asstatic random access memory (SRAM), dynamic random access memory (DRAM),synchronous DRAM (SDRAM), read-only memory (ROM), a combination thereof,or the like. In an embodiment, the memory 608 may include ROM for use atboot-up, and DRAM for program and data storage for use while executingprograms.

The mass storage 604 may comprise any type of storage device configuredto store data, programs, and other information and to make the data,programs, and other information accessible via the bus 620. The massstorage 604 may comprise, for example, one or more of a solid statedrive, hard disk drive, a magnetic disk drive, an optical disk drive, orthe like.

The video adapter 610 and the I/O interface 612 provide interfaces tocouple external input and output devices to the processing unit 602. Asillustrated, examples of input and output devices include a display 618coupled to the video adapter 610 and a mouse/keyboard/printer 616coupled to the I/O interface 612. Other devices may be coupled to theprocessing unit 602, and additional or fewer interface cards may beutilized. For example, a serial interface such as Universal Serial Bus(USB) (not shown) may be used to provide an interface for a printer.

The processing unit 602 also includes one or more network interfaces606, which may comprise wired links, such as an Ethernet cable or thelike, and/or wireless links to access nodes or different networks. Thenetwork interfaces 606 allow the processing unit 602 to communicate withremote units via the networks. For example, the network interfaces 606may provide wireless communication via one or more transmitters/transmitantennas and one or more receivers/receive antennas. In an embodiment,the processing unit 602 is coupled to a local-area network 622 or awide-area network for data processing and communications with remotedevices, such as other processing units, the Internet, remote storagefacilities, or the like.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A method of loop suppression in a layer-two transit network with multiprotocol label switching (MPLS) encapsulation, comprising: receiving a packet at a provider edge (PE) router for the layer-two transit network; storing the packet according to a packet data structure in a non-transitory memory, wherein the packet data structure includes an MPLS label field and a layer-two header; determining a time-to-live (TTL) attribute for the packet; writing the TTL attribute to the non-transitory memory in the MPLS label field; and routing the packet according to information in the layer-two header.
 2. The method of claim 1 wherein the routing includes transmitting the packet to a provider router in the layer-two transit network according to a destination address (DA) and a virtual local area network identifier (VID) in the layer-two header.
 3. The method of claim 1 wherein the routing includes transmitting the packet to a provider router in the layer-two transit network according to a backbone media access control (BMAC) address in the layer-two header.
 4. The method of claim 1 wherein the determining includes calculating a maximum number of hops to traverse the layer-two transit network.
 5. The method of claim 4 wherein the determining includes calculating a TTL start value linearly related to the maximum number of hops.
 6. The method of claim 4 wherein the TTL attribute is equal to the maximum number of hops.
 7. The method of claim 1 wherein the storing the packet includes writing the MPLS label field to a memory block having an MPLS label portion, a quality of service (Qos) portion, a stack indicator portion, and a TTL portion.
 8. The method of claim 7 wherein the TTL portion of the non-transitory memory includes an 8-bit field.
 9. The method of claim 1 wherein the MPLS label field comprises an MPLS transit label field and the writing the TTL attribute further comprises writing the TTL attribute to the non-transitory memory in the MPLS transit label field.
 10. The method of claim 1 wherein the MPLS label field comprises an MPLS service label field and the writing the TTL attribute further comprises writing the TTL attribute to the non-transitory memory in the MPLS service label field.
 11. A method of Ethernet packet routing in a layer-two transit network with multiprotocol label switching (MPLS) encapsulation, comprising: receiving a packet at a provider router for the layer-two transit network, wherein the packet has an MPLS label field and a layer-two header; storing the packet in a non-transitory memory; and evaluating a time-to-live (TTL) attribute in the MPLS label field for loop detection by: dropping the packet when the TTL attribute indicates a loop, and recalculating and writing the TTL attribute to the non-transitory memory and routing the packet according to information in the layer-two header, when the TTL attribute does not indicate the loop.
 12. The method of claim 11 wherein the routing includes transmitting the packet to another provider router in the layer-two transit network according to a destination address (DA) and a virtual local area network identifier (VID) in the layer-two header.
 13. The method of claim 11 wherein the routing includes transmitting the packet to another provider router in the layer-two transit network according to a backbone media access control (BMAC) address in the layer-two header.
 14. The method of claim 11 wherein the storing the packet includes writing the MPLS label field in a memory block having an MPLS label portion, a quality of service (QoS) portion, a stack indicator portion, and a TTL portion.
 15. The method of claim 11 wherein the TTL attribute indicates the loop when its value reaches a TTL end value.
 16. The method of claim 11 wherein the recalculating the TTL attribute includes computing a new TTL value according to a linear function of a current value of the TTL attribute.
 17. The method of claim 16 wherein the linear function comprises decrementing the current value by one.
 18. The method of claim 11 wherein the MPLS label field comprises an MPLS transit label field and the writing the TTL attribute further comprises writing the TTL attribute to the non-transitory memory in the MPLS transit label field.
 19. The method of claim 11 wherein the MPLS label field comprises an MPLS service label field and the writing the TTL attribute further comprises writing the TTL attribute to the non-transitory memory in the MPLS service label field.
 20. A provider edge (PE) router, comprising: a network interface controller (NIC) couplable to a transit network; a non-transitory memory configured to store a multiprotocol label switching (MPLS) encapsulated packet having an MPLS label field and a layer-two header; and a processor coupled to the non-transitory memory and the NIC and configured to: compute a starting time-to-live (TTL) value for the MPLS encapsulated packet, write the starting TTL value to the MPLS label field in the non-transitory memory, and instruct the NIC to transmit the MPLS encapsulated packet according to information in the layer-two header.
 21. The PE router of claim 20 wherein the non-transitory memory is further configured to store the MPLS encapsulated packet according to a packet data structure having an MPLS transit label portion, and wherein the processor is further configured to write the starting TTL value to the MPLS transit label portion.
 22. The PE router of claim 20 wherein the non-transitory memory is further configured to store the MPLS encapsulated packet according to a packet data structure having an MPLS service label portion, and wherein the processor is further configured to write the starting TTL value to the MPLS service label portion.
 23. The PE router of claim 20 wherein the non-transitory memory is further configured to store the MPLS encapsulated packet according to a packet data structure having an MPLS transit label portion and an MPLS service label portion.
 24. The PE router of claim 23 wherein the processor is further configured to write the starting TTL value to the MPLS transit label portion.
 25. The PE router of claim 20 wherein the non-transitory memory is further configured to store the MPLS label field of the MPLS encapsulated packet according to an MPLS label data structure having an MPLS label portion, a quality of service (QoS) portion, a stack indicator portion, and a TTL portion.
 26. The PE router of claim 20 wherein the processor comprises an application specific integrated circuit (ASIC) coupled to the NIC and the non-transitory memory.
 27. The PE router of claim 20 wherein the processor is further configured to compute the starting TTL value according to a maximum number of hops for traversing the transit network.
 28. The PE router of claim 20 wherein the processor is further configured to instruct the NIC to transmit the MPLS encapsulated packet according to a backbone media access control (BMAC) address in the layer-two header stored in the non-transitory memory.
 29. A provider router, comprising: a network interface controller (NIC) having a first port and a second port, both couplable to a transit network; a non-transitory memory configured to store a multiprotocol label switching (MPLS) encapsulated packet receivable through the first port, wherein the MPLS encapsulated packet comprises an MPLS label field and a layer-two header; and a processor coupled to the NIC and the non-transitory memory and configured to: evaluate a time-to-live (TTL) attribute stored in a memory block of the non-transitory memory corresponding to the MPLS label field for loop detection, drop the MPLS encapsulated packet when the TTL attribute in the non-transitory memory indicates a loop, and compute a new TTL value for the TTL attribute and cause the new TTL value to be written to the memory block corresponding to the MPLS label field when the TTL attribute in the non-transitory memory does not indicate the loop, and cause the NIC to transmit the MPLS encapsulated packet through the second port according to information in the layer-two header.
 30. The provider router of claim 29 wherein the MPLS label field comprises an MPLS transit label field, and wherein the memory block of the non-transitory memory corresponds to the MPLS transit label field.
 31. The provider router of claim 29 wherein the MPLS label field comprises an MPLS service label field, and wherein the memory block of the non-transitory memory corresponds to the MPLS service label field.
 32. The provider router of claim 29 wherein the non-transitory memory is further configured to store the MPLS encapsulated packet according to a packet data structure having an MPLS transit label portion and an MPLS service label portion.
 33. The provider router of claim 32 wherein the processor is further configured to: evaluate the TTL attribute stored in the MPLS transit label portion; and cause the new TTL value to be written to the MPLS transit label portion.
 34. The provider router of claim 29 wherein the non-transitory memory is further configured to store the MPLS label field of the MPLS encapsulated packet according to an MPLS label data structure having an MPLS label portion, a quality of service (QoS) portion, a stack indicator portion and a TTL portion.
 35. The provider router of claim 29 wherein the processor comprises an application specific integrated circuit (ASIC) coupled to the NIC and the non-transitory memory.
 36. The provider router of claim 29 wherein the processor is further configured to instruct the NIC to transmit the MPLS encapsulated packet according to a backbone media access control (BMAC) address in the layer-two header stored in the non-transitory memory.
 37. The provider router of claim 29 wherein the processor is further configured to compute the new TTL value according to a linear function of a current value of the TTL attribute.
 38. The provider router of claim 37 wherein the linear function comprises decrementing the current value of the TTL attribute by one.
 39. The provider router of claim 29 wherein the processor is further configured to compare a current TTL value of the TTL attribute to an end TTL value to determine when to drop the MPLS encapsulated packet. 